1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory. In particular, the present invention relates to an NMOS transistor type nonvolatile semiconductor memory.
2. Description of Related Art
Electrically erasable and programmable nonvolatile semiconductor memories such as flash memories are known. Typically, a memory cell of such nonvolatile semiconductor memories has a structure of an NMOS transistor. More specifically, an NMOS transistor type nonvolatile memory cell includes a “charge storage layer” which is provided between a gate electrode (control gate) and a P-type semiconductor substrate. As examples of the charge storage layer, a floating gate or a nitride film of an ONO (Oxide Nitride Oxide) film are used. A threshold voltage of the NMOS transistor type nonvolatile memory cell varies depending on the amount of charges stored in the charge storage layer. Use of this variation of the threshold voltage allows the nonvolatile memory cell to nonvolatily store data.
There is known a technique (HHI: Hot Hole Injection) of injecting hot holes to the charge storage layer in order to lower the threshold voltage of the NMOS transistor type nonvolatile memory cell. Yutaka Hayashi, Seiki Ogura, Tomoya Saito, and Tomoko Ogura, “Twin MONOS Cell with Dual Control Gates”, 2000 Symposium on VLSI Technology Digest of Technical Papers, pp. 122-123, 2000 discloses a technique that a “Band-to-Band tunnel phenomenon” causes hot holes near an N-type diffusion layer. Then, the hot holes having high energy go over a potential barrier of an oxide film in the ONO film to be injected into a nitride film in the ONO film. The Hot Hole Injection is also referred to in Japanese Patent Laid Open Publication No. 2001-512290.
In general, a nonvolatile semiconductor memory is formed on a silicon wafer. On the silicon wafer, a crystal plane (100) of a silicon crystal is set as a surface on which an NMOS transistor type nonvolatile memory cell is formed. This is because the interface state between the silicon and the oxide film is the smallest in the case of the crystal plane (100). In addition, the nonvolatile semiconductor memory is generally formed along a crystal orientation <110>. This is because it is easiest to cut the silicon wafer along the crystal orientation <110> from the view point of a cleavage property when rectangular devices are cut from the silicon wafer. Thus, a channel direction of the nonvolatile memory cell is generally congruent with a crystal orientation <110> of the silicon.
Japanese Patent Application Laid-open Publication No. 2002-305291 discloses a technique relating to a crystal plane and crystal orientation of a silicon wafer. More specifically, in '291 publication, there is disclosed a technique for reducing metallic contamination in an SOI (Silicon On Insulator) substrate when devices are manufactured on the SOI substrate. According to the technique, a base substrate and a bond substrate, on the latter of which the devices are to be formed, are firstly prepared in order to form the SOI substrate. Both of the base substrate and the bond substrate are single crystal silicon wafers whose principal surfaces are crystal planes (100). Within the surface of the base substrate, notches are formed in a crystal orientation <011>, whereas within the surface of the bond substrate, notches are formed in a crystal orientation <010>. The base substrate and the bond substrate are bonded via an oxide film in such a manner that these notches overlap each other. Thus, the SOI substrate is created. In this manner, the base substrate and the bond substrate are bonded so as to have different crystal orientations. As a result, the gettering ability of the SOI substrate is improved. A device such as a flash memory is formed on the SOI substrate so that a longitudinal direction thereof would be parallel or vertical to the crystal orientation <010> of the bond substrate. Thus, the gettering ability of the SOI substrate is improved and the metallic contamination is suppressed, so that reliability of the device is improved.